1. Field of Invention
This invention relates to laser gyros, and more particularly to dispersion compensation therefor.
2. Description of the Prior Art
There are many forms of laser gyro known to the art. A typical laser gyro has waves of the same nominal frequency and polarization circulating in both the clockwise and counterclockwise directions, a rotation of the laser causing the wave traveling in the direction of rotation to assume a lower frequency than the wave traveling in the direction opposite to that of the rotation, whereby a difference in frequency of the two waves is an indication of the rotation being sensed. Since there is a tendency of the counter-rotating waves to lock together at the same frequency, when the rotation rate is small and the frequency differential is therefore small, it has been known to provide effective rotational bias to the laser, either in the form of mechanical bias or Faraday effect bias (a magnetic field operating on a transparent optical material, such as glass or quartz, in the laser cavity), of a sufficient magnitude such that the two waves are always operating at a frequency differential which is too great to permit frequency locking of the two waves. In some gyros, the polarity of the rotational bias is periodically reversed so that the bias is eliminated from the time-average gyro output.
C. In another form of laser gyro, the bias is eliminated from the output by having essentially two laser gyros operating in the same optical path, one with right circular (or elliptical) polarization, and the other with left circular (or elliptical) polarization, the bias being the same for opposite polarizations, whereby it is cancelled by differencing the outputs, without reversing bias. Such a gyro, referred to as a differential laser gyro, is disclosed in a commonly owned copending application to Yntema et al, Ser. No. 763,277, filed on Sept. 27, 1968, now U.S. Pat. No. 3,862,803 entitled DIFFERENTIAL LASER GYRO. The differential laser gyro has the advantage that bias switching asymmetries are obviated since only a single polarity of bias is utilized, and under ideal conditions any extraneous magnetic effect (such as the earth's magnetic field) operates equally on both polarizations, and is therefore substantially cancelled.
Other gyros may not use bias polarity reversal and cancellation, but rather simply rely on subtraction of the known nominal bias magnitude from the result obtained.
As is known, the gain versus frequency characteristic of the laser gain medium always has associated therewith an index of refraction, or dispersion characteristic, which provides a slightly different optical length through the laser gain medium for waves of different optical frequencies; e.g. for clockwise vs counterclockwise waves when they are biased to different frequencies as above. The effect is opposite in dependence upon the polarity of the bias (that is, whether the clockwise or counterclockwise wave is operating at a higher or lower frequency, and therefore at a higher or lower point on the dispersion characteristic). In gyros where bias is reversed for bias cancellation purposes, this difference in the optical length is cancelled along with the applied bias. However, in gyros which do not employ bias reversal, and in the differential laser gyro wherein bias cancellation is automatically effected by the combination of outputs from opposite polarizations, the variations which can occur in the effective optical length for the counter-rotating waves are not cancelled. These variations result from the fact that, due to minor shifts in the optical length of the total optical cavity of the laser gyro, the absolute frequency of both counter-rotating waves may increase together, or decrease together, and since the dispersion characteristic is nonlinear characteristic, this can cause the additional variation of optical length within the gain medium to vary as between the two waves, thereby providing the same effect as the change in the rotational rate of the gyro, which introduces significant errors in the rate sensed. Similar errors can also result from fluctuations or perturbations of the gain medium itself.